The invention relates to a method for burning carbonate-containing material in a shaft kiln, with gravity conveying through a preheating zone, at least on e burning zone and a cooling zone to a discharge device, fuel supply in the burning zone or adjacent thereto taking place by means of several burning lances passed through the shaft wall and combustion air is supplied under overpressure as cooling air.
Particularly when burning small-grain material, i.e. in the case where a significant proportion of the material to be burned has a grain size of less than 30 mm, the problem arises of supplying in uniform manner to the material the necessary heat quantity, so that each grain can be burned through to its core without the grains sintering together as a result of local overheating and forming solid bridges in the kiln. This problem is particularly pronounced if higher degrees of burning are needed over and beyond soft burned products.
For small-grain burning material and a uniform burning and therefore product quality, it is most appropriate to use rotary kilns, because an intense material circulation ensures a good and uniform heat transfer to each grain or particle. However, it is disadvantageous that they have a very complicated and costly construction and to the correspondingly high capital expenditure must be added the high operating costs resulting from significant levels of wear and high heat losses due to radiation and waste gases, which have a particularly marked effect when using higher temperatures such as are necessary for higher burning levels or other product qualities, such as medium burned, hard burned and sintered products.
Another method uniformly supplying to the product being burned the heat quantity necessary for burning purposes consists of the admixing of fuel, i.e. metallurgical coke to the product being burned in mixed firing kilns. However, mixed firing kilns are unsuitable for small-grained burning material. They also suffer from the significant disadvantage that the ash resulting from the burning of coke remains in the completely burned product and consequently leads to a lower, grey coloured product quality.
An energy-saving operation results from multi-shaft kilns based on parallel flow-regenerative methods using so-called MAERZ kilns. The fuel is supplied in such kilns by burning lances immersed in suspended manner in the material being burned and which are uniformly distributed over the shaft cross-section in the charging zone. However, such known kilns are only suitable for soft burned products.
U.S. Pat. No. 5,460,517 describes how it is possible to burn small-grain burning material by a particular grain size distribution during kiln charging, combined with a special design of the shaft chambers.
If fuel quantities suitable for hard burned products is to be supplied to the burning zone of a shaft kiln, in order to obtain the burning temperatures necessary, hitherto insurmountable difficulties have occurred with regards to obtaining a uniform temperature distribution over the shaft cross-section and in particular preventing a sintering together of burning material as a result of local overheating.
U.S. Pat. No. 4,094,629 proposes reducing the width of the shaft cross-section through an annular construction thereof and to place additional burner orifices in the inner wall obtained. In this way it is possible to maintain a uniform downward movement of the burning material under gravity without the material flow being disturbed by fittings in the shaft.
Fittings in the form of beam-like burner supports are described in GB-A-1111746, which as a result of receiving e.g. in each case twenty liquid-cooled burners have a relatively wide cross-section and consequently bring about a significant reduction in the useful kiln cross-section, associated with the risk of local blocking of gravity conveying of the burning material.
A summary description of various burning methods, including the aforementioned burning in regenerative multi-shaft kilns appears in the handbook xe2x80x9cChemistry and Technology of Lime and Limestonexe2x80x9d, Robert S. Bynton, second edition, 1987.
The problem of the invention is to find a method of the aforementioned type making it possible to burn in particular small-grained burning material with different degrees of burning and extending to dead burning in an economic manner in shaft kilns so as to bring about a high quality product.
According to the invention this problem is solved by a method of the aforementioned type and which is characterized in that the supply of fuel takes place by means of numerous burning lances displaceable into the shaft chamber and positioned perpendicularly to the shaft wall through the choice of the position of their orifices in such a way that the individual flames formed at the lances together form a flame area, which at least approximately extends over the entire shaft cross-section.
As each burning lance preferably is intended to only form a single flame, compared with burner supports having numerous burners it has a limited cross-section and consequently only leads to an insignificant influencing of the burning material flow. It has surprisingly been found that the burning lances still have an adequate bending strength to absorb the pressure of the granular burning material flowing round them. Preferably the grain size of the burning material is limited to 70 mm.
As a result of the extension of each burning lance perpendicular to the shaft wall it is ensured that between the said lance and the shaft wall no gap is formed in which the burning material could accumulate. The local restriction to the shaft cross-section through the burning lances projecting into it can be reduced by arranging the burning lances in several superimposed planes circumferentially displaced with respect to those of another plane, so that the necessary fuel quantity is supplied distributed over several shaft planes.